1. Field of the Invention
The present invention relates to a technical field of wireless communications. More particularly, the present invention relates to a wireless communication terminal and a wireless communication method in a mobile communication system conforming to a carrier sense multiple access (CSMA) scheme.
2. Description of the Related Art
In the CSMA scheme, a terminal performs virtual carrier sense before sending data so as to determine whether another terminal is communicating with a base station (also referred to as an access point). When the another terminal is communicating with the base station, the terminal transmits its wireless packet after the another terminal completes the communication. In principle, every terminal fairly has a right to transmit a packet. The virtual carrier sense is performed in the following way as a whole.
First, the terminal generates a random number within a predetermined CW (Contention Window) range after a channel remains in an idle state for a period called IFS (Inter Frame Space). Then, a random time period is determined based on the random number so that back-off control is performed within the random time period. The idle state indicates a state in which the wireless section is not used. In the back-off control, the random number is set to be an initial value, and the initial value is decreased as time passes, so that a wireless packet is actually transmitted when the number becomes 0.
The ISF is defined in the wireless local area network (LAN) standard IEEE802.11, and is a time period of an idle state that should be detected before packet transmission. The CW is a maximum value of the random number in the back-off control, and is a parameter necessary for realizing user multiplicity. In the IEEE802.11, a minimum CW value CWmin and a maximum CW value CWmax are defined. In back-off in a first transmission, the random number is generated using the CWmin, and each time when retransmission is performed, the value of the random number is increased to be doubled so that the back-off is performed. By the way, the CWmax is an upper limit value of CW. By gradually increasing the value of CW, collision frequency and total traffic amount (including first transmission and retransmissions) can be decreased steadily. However, there is a possibility in that a plurality of terminals transmits wireless packets simultaneously in this scheme. When packet collision occurs, communication quality may be degraded and delay due to retransmission may continue for a long time.
Following documents are related to the technical field of the present invention.                [Patent Document 1] Japanese Laid-Open Patent Application No. 2005-12725        [Patent Document 2] Japanese Laid-Open Patent Application No. 2005-184522        [Non-Patent Document 1] ANSI/IEEE std 802.11, Wireless LAN medium access control(MAC) and physical layer (PHY) specifications, 1999        
The non-patent document 1 discloses a technique to handle the problem by performing certain priority control. In this technique, EDCA (Enhanced Distributed Channel Access) that is defined in IEEE802.11e is used. Each packet is provided with a priority of four types of priorities, and each of IFS, CWmin and CWmax for a high priority packet is set to be short so that the high priority packet is transmitted preferentially. Accordingly, relative priority control can be performed in a communication environment in which various applications are mixed such as a voice communication application and a data communication application.
Also, as described in the non-patent document 1, in HCCA (HCF controlled channel access) defined in IEEE802.11e, a base station transmits a polling frame including channel use time period (TXOP: Transmission Opportunity) information to a communication terminal, and the communication terminal that receives the information obtains a preferential transmission right without the back-off.
The patent document 1 discloses a technique in which a base station reports a parameter related to transmission waiting time to each terminal according to a traffic amount in the whole of a cell, so that each terminal performs transmission according to the parameter. In this technique, it is intended to reduce possibility of collision of packets.
The patent document 2 discloses a technique in which transmission is allowed only at a transmission available time that is assigned within a predetermined time that repeats at a frequency. In this technique, it is intended that each terminal can perform communications smoothly.
However, in the conventional method such as the EDCA, merely a relative transmission priority order between packets having different priorities is determined, but guarantee of quality or contention avoidance is not performed between terminals that try to transmit packets having the same priority.
Therefore, in a case where a plurality of terminals transmit packets having the same priority simultaneously, the possibility that collision of packets occurs cannot be reduced. When the collision of packets occurs, even if send and receive of the packet are successfully performed after the collision occurs by performing retransmission, a delay occurs due to the retransmission. In addition, if the retransmission also fails, discard of packet occurs so that there is a possibility in that unrecoverable signal quality deterioration may occur.
In a case where the back-off control is performed for obtaining packet transmission right by a plurality of terminals, a terminal from which transmission right is deprived of by another terminal needs to wait in order to obtain another transmission right so that delay increases. The more the number of terminals is, the higher an occurrence probability of the problem becomes. In a real-time application such as the VoIP (Voice over Internet Protocol), such delay and packet loss may cause especially critical quality deterioration.
On the other hand, in the HCCA, channel use time period is provided by sending the polling frame to each terminal so that signal quality guarantee may be provided. However, a concrete schedule designing method is not clearly defined, and also decrease of the transmission rate due to retransmission timing or link adaptation is not considered. In the application such as the VoIP in which it is necessary to periodically transmit a packet, when once collision of polling occurs collision may occur every time after that, so that the problem of signal quality degradation becomes serious. In addition, as to the HCCA, different from the EDCA, the HCCA requires that the terminal is provided with a specific chipset. Therefore, when adopting the HCCA scheme, it is necessary to provide a device supporting the HCCA in both of the terminal and the base station. Therefore, it is concerned that introduction of this method to an existing system is not easy.
In the technique disclosed in the patent document 1, although the base station dynamically changes the parameter on the transmission waiting time for each terminal according to the traffic amount of the whole of the cell, there remain above-mentioned concerns relating to contention of packets and quality deterioration since transmission timing is not provided to each terminal based on certain priority. In the patent document 2, each terminal may set transmission available time, but any concrete method is not disclosed. Especially, methods are not disclosed for the following cases: when retransmission is performed, when send/receive occupation period is changed due to transmission rate decrease, when there is a terminal that includes the function of the patent document 2 and another terminal that does not include the function of the patent document 2 (especially when packet contention occurs between the terminal that includes the function of the patent document 2 and another terminal that does not include the function of the patent document 2).